In order to transmit or store television signals efficiently, the signal is often digitized and coded. In the case of digitization, the television signal is sampled so that discrete pixels are obtained, each represented by a number of bits. In the case of coding, the pixels are converted into code words for transmission or storage. The coding causes the number of bits required for transmitting or storing the code words to be considerably smaller than the number of bits which is required for transmitting the original pixels. The television signal is subsequently reconstructed from the code words as satisfactorily as possible. To realize a picture quality which is as satisfactory as possible at a given bit rate, the digital television signal is processed prior to coding. An example of processing television signals is low-pass filtering which is used to reduce the resolution of the television picture.
A device for processing interlaced television signals is known from Reference [1]. In this known device, a signal having a reduced resolution is derived from a television signal by means of processing. More specifically, a compatible standard television (SDTV) signal is derived from a high-definition television (HDTV) signal. The SDTV signal is obtained by applying the HDTV signal to a vertical low-pass filter. The output signal of the vertical low-pass filter thus comprises a low-frequency portion of the input signal in the vertical direction and is hereinafter referred to as the baseband signal or spatial signal. After it has been subjected to a horizontal low-pass filtering, this signal may be displayed on standard television receivers. As described in the Reference, the SDTV signal could also be obtained by intraframe processing of the video signal in which the vertical filtering is applied to each HDTV frame. However, unacceptable motion artefacts then occur. As will be explained hereinafter, these motion artefacts are an inherent result of the vertical filtering. Therefore, field processing is used in the known device. This means that the two interlaced fields constituting a television frame are filtered separately. However, as described in the Reference, field processing increases the bit rate. Moreover, different vertical filters appear to be required for the two fields, which increases the complexity of such a compatible transmission system.